1. Field of the Invention
The present invention relates to a hue compensation apparatus and, in particular, to a hue compensation apparatus for a color hard copy machine.
2. Description of the Related Art
Often, it is desirable to keep a copy of a CRT (cathode ray tube) display in the form of a photograph or a hard copy. For example, the CRT display may illustrate characteristic curve data in an experiment, figures in CAD (computer aided design) equipment, or TV displays. Color hard copy machines are developed to respond to the needs mentioned above. Many types of color hard copy machines exist; a sublimation-type color hard copy machine is described below.
In the sublimation-type color hard copy machine, first, color signals R (red), G (green) and B (blue), which are derived from picture signals of pictures displayed on the CRT, are transformed to color signals Y (yellow), M (magenta) and C (cyan). Then, heat according to the signals Y, M and C is supplied to thermal print paper by printing heads for every color signal Y, M and C. A color hard copy of the picture displayed on the CRT is obtained by the above procedure.
To properly reproduce the color of the picture displayed on the CRT, compensation as described below is required. That is, the difference between the characteristics of signals R, G, B and signals Y, M, C, or the non-linearity of color depth development to the thermal energy supplied to the thermal print paper, or the non-linearity of color development characteristics in the thermal print paper, or the coming out of a part of a color developed first when next color development is implemented in groups after first color development, need to be compensated. Thus, hue compensation to signals R, G, B need to be implemented to obtain an accurate color hard copy in relation to the original picture.
FIG. 1 is a block diagram of one example of a hue compensation apparatus according to the prior art. Input terminals 10, 12 and 14 receive picture data R, G and B, respectively, to be printed. An input terminal 16 receives a color selection signal which is described later. A lookup table circuit 18 is connected to input terminals 10, 12, 14, 16. One of signals Y, M and C, for example, Y, which is hue-compensated according to input signals R, G and B from input terminals 10, 12 and 14, is output from lookup table circuit 18. The color of output signals from lookup table circuit 18 is designated by the color selection signal from input terminal 16. An output terminal 20 is connected to lookup table circuit 18 and outputs one of signals Y, M and C hue-compensated which is designated by the color selection signal from input terminal 16 as mentioned above.
If signals input to input terminals 10, 12 and 14 are 8 bit signals, the capacity of memory in lookup table circuit 18 must be 16M bites (=2.sup.8 .times.2.sup.8 .times.2.sup.8) per color. To print three colors, lookup table 18 needs 48M bites (=16M bites.times.3) of memory capacity. The signal for color selection input to input terminal 16 needs only 2 bits.
In the embodiment of FIG. 1, the data must be stored in lookup table circuit 18 which outputs signals Y, M and C responsive to a pair of input signals R, G and B. Moreover, storing the data (which has 2.sup.24 possible combinations) in lookup table circuit 18 is also cumbersome.
FIG. 2 is a block diagram of another example of a hue compensation apparatus according to the prior art. In FIG. 2, input terminals 22, 24 and 26 receive color signals R, G, B, respectively. An operation circuit 28 is connected to input terminals 22, 24 and 26. Operation circuit 28 performs a matrix operation as follows: ##EQU1## Output terminals 30, 32 and 34 are connected to operation circuit 28, and receive the output signals R.sub.HC, G.sub.HC, B.sub.HC which are hue-compensated, respectively, from operation circuit 28.
In the apparatus, hue-compensated signals R.sub.HC, G.sub.HC, B.sub.HC are obtained from output terminal 30, 32 and 34, respectively, according to the input signals R, G, B from input terminals 22, 24 and 26.
The apparatus contains simple hardware and is low in cost. However, coefficients multiplied to input signals R, G, B are constant. Thus, the hue compensation method as described may not compensate for non-linear color development characteristics of the thermal print paper. Accordingly, there is a drawback that the hard copy picture may be different from the picture displayed on the CRT.